The ABCs of Roof Fire Ratings

Does your roof meet code and insurance requirements?

By Richard L. Fricklas

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Non-combustible roof desks, such as steel, poured gypsum, or concrete, are tested by evaluating how quickly flame spreads on the top surface of the roof system. Combustible decks are submitted to two additional tests after this.

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Combustible decks containing materials such as wood, plywood, or oriented strand board are subjected to extra test methods, including the burning brand test, which involves placing an ignited wood lattice over the test specimen. Failure is reached when the roof system ignites. Varying weights of lattices are used to determine a Class A, B, or C rating.

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FM Global and UL both use fire resistance tests developed in the aftermath of a major interior building fire in Livonia, MI, in 1953. The building in question was thought to be highly fire resistant, but failed catastrophically. The 30-acre building burned to the ground.

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This test building, dubbed the “White House,” was a 100-foot-long replica of the Livonia structure. The fire burned from end to end in 10 minutes, an unacceptable result.

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The Steiner tunnel is a 25-foot test chamber that allows UL to measure the spread of flames on the underside of the roof deck.

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Flames cannot progress more than 10 feet down the Steiner tunnel in 10 minutes and 14 feet during the entire 30-minute test. Successful results are classified as insulated metal decks.

Roofing systems must meet or exceed building codes and insurance requirements. Fire ratings may pertain to resistance to fire from above the roof system (the familiar Class A, B, or C ratings), or fire exposure from the building interior (the underside of the roof deck).

How Are Roofs Rated?ASTM E108 defines fire test methods for roof coverings. These tests may be conducted at UL Inc., FM Global, or any other certified testing laboratory. E108 defines the following conditions:

Richard (Dick) L. Fricklas

Richard (Dick) L. Fricklas was technical director emeritus of the Roofing Industry Educational Institute prior to his retirement. He is co-author of The Manual of Low Slope Roofing Systems, and continues to participate in seminars for the University of Wisconsin and RCI Inc.-The Institute of Roofing, Waterproofing, and Building Envelope Professionals. His honors include the William C. Cullen Award and Walter C. Voss Award from ASTM, the J. A. Piper Award from NRCA, and the James Q. McCawley Award from the MRCA. Dick holds honorary memberships in both ASTM and RCI Inc.

Over non-combustible roof decks such as steel, poured gypsum, or concrete, only the spread of flame on the top surface of the roof system is evaluated. The maximum flame spread is 6 feet for a Class A rated roof, 8 feet for Class B, and 13 feet for Class C. The slope of the test specimen is preselected, and since steeper slopes are more of a challenge due to melting material feeding the fire, the rating applies to the maximum slope passed.

Gravel-surfaced built-up roofing and ballasted single-ply systems usually meet Class A, while mineral surfaced roofings may be Class B. Some unsurfaced systems, such as asphalt glazing, may be Class C or even unrated.

If the roof deck is combustible, such as wood, plywood, or OSB, two additional tests must be conducted. Both are burn-through tests, and the ultimate roof systems rating is the lowest of the three tests.

The burning brand: The brands consist of an ignited wood latticeplaced directly on top of the test specimen. The Class A brand weighs 2,000 grams, Class B 500 grams, and Class C 9-1/4 grams. Failure is defined as the point where the roof deck ignites.

The intermittent flame test: A gas flame is cycled on and off. To meet Class A, the test specimen must resist 15 cycles, Class B eight cycles, and Class C three cycles.

Fire resistance ratings (time-temperature) include structural elements and everything above, including the roof membrane and its surfacing. A one-hour rating would mean the structural elements have not yet reached their yield point when exposed to a under-deck heat load defined in ASTM E119 for steel structural members as reaching 1,070 degrees F.

Interior fire hazard is evaluated by FM Global and/or UL using differing test procedures. Both relate to a major interior building fire in Livonia, MI, back in 1953. In that case, an insulated steel building that might be expected to be highly fire-resistant failed catastrophically. Heat from an under-deck fire melted and vaporized the asphalt used to adhere the thermal insulation to a steel deck, feeding the fire. Some 30 acres of building burned to the ground.

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